Method for the production of metallic heat transfer bodies



July 26, 1966 w. ROSTOKER ET AL 3,262,190

METHOD FOR THE PRODUCTION OF METALLIC HEAT TRANSFER BODIES OriginalFiled July 10, 1961 /0 4 l INVENTORS F MZM/V 2057016? //4 BY 246KB) ,lzp

United States Patent 3,262,190 METHOD FOR THE PRODUCTION OF METALLICHEAT TRANSFER BODIES William Rostoker, Chicago, and Robert H. Read,Chicago Heights, Ill., assignors to IIT Research Institute, acorporation of Illinois Original application July 10, 1961, Ser. No.122,844. Divided and this application Apr. 21, 1965, Ser. No. 476,770

5 Claims. (Cl. 29-1573) This application is a division of our copendingapplication Serial No. 122,844 filed July 10, 1961.

The present invention is directed to improved heat transfer systems ofthe type employed, for example, in automobile radiators, heaters,refrigerators, and air conditioning systems. The heat transfer systemsof the present invention are specifically designed to replace theconventional fin and tube structures now commonly employed as heatexchange elements.

The manufacture of fin and tube type heat exchangers is usuallyaccomplished by an assembly of blanked and stacked fins strung overserpentine heat transfer tubes. While such assemblies are reasonablyeificient heat transfer systems, they are rather difficult to assembleand consequently are relatively expensive to manufacture.

The present invention employs techniques of the new field of fibermetallurgy in building up a heat transfer system consisting basically ofa heat transfer element having relatively short, heat conductive fibersof metal bonded to the surface of the heat transfer element and bondedto each other along their areas of contact. This structure results in animproved heat transfer system because of the very large surface tovolume character of the metal fibers. The improved heat transfer systemis also capable of being automated more completely than present methodsand therefore provides a cheaper manufacturing cost by virtue of reducedlabor cost.

Fiber metallurgy concerns itself with the manufacture and use ofmetallic fibers, that is, metallic elements whose length is considerablygreater than any dimension in cross-section but is not so long as toconstitute a continuous filament. As a general rule, the fiber has aratio of at least to 1 between its length and its mean dimension incross-section. In the case of a circular fiber, the mean dimension isthe diameter, while in the case of a rectangular fiber, the meandimension is one-half the sum of the short side and the long side of therectangle.

When metallic fibers of the character described are suitably depositedby any of a variety of methods to be described later, they assume arandom three dimensional distribution which provides a uniform porosity,and remarkable strength to porosity ratios. The strength characteristicsof the fibers arise from providing metal-tometal bonds between thefibers along their areas of contact. Such metal-to-metal bonds may beprovided, for example, by sintering the fibers at an appropriatesintering temperature, or by employing pre-coated fibers having acoating of a brazing material thereon and then heating the fibers to atemperature sufficient to melt the brazing material without melting thefibers, causing the molten brazing material to eventually solidify atthe points of contact between the fibers and bond them together.

The unique strength to porosity ratio, the ability to produce extremelyporous materials, and the very large surface to volume character of thedeposited fiber mass are properties which adapt such fiber metallurgystructures to the field of heat transfer elements.

Accordingly, an object of the present invention is to provide a methodfor the production of an improved heat transfer system utilizing ahighly porous heat transfer means.

Another object of the invention is to provide a method for theproduction of a highly porous felted heat transfer element for use inheat transfer systems.

Still another object of the invention is to provide an improved methodfor assembling a heat exchange device.

Another object of the invention is to provide a method for themanufacture of heat transfer elements which is more readily adaptable toautomation and is less expensive than methods presently used in themanufacture of fin and tube type heat transfer elements.

A further description of the present invention will be made inconjunction with the attached sheet of drawings in which:

FIGURE 1 is a plan view of the heat transfer assembly in an early stageof formation;

FIGURE 2 is a cross-sectional view taken substantially along the line11-11 of FIGURE 1;

FIGURE 3 is a view similar to FIGURE 2 but illustrating the heattransfer assembly with the metallic fibers incorporated therein andbonded together;

FIGURE 4 is a plan view of the finished assembly; and

FIGURE 5 is a view in perspective of a. modified form of the invention.

As shown in the drawings:

In FIGURE 1, reference numeral 10 indicates generally an open supportframe consisting of sheet metal or the like. The frame 10 carries aconventional serpentine type tube 11 consisting of copper or the likeand having its ends 11a and 11b secured to the frame 10. A relativelycoarse metal screen 12 is fastened to one side of the frame 10 torigidify the frame 10 and also to serve as a collector for the metalfibers which are subsequently deposited over and about the tube 11.

Relatively small, heat conductive fibers are then deposited in the formof a felt over the tube 11 so that the tube is completely immersedwithin a mat 13 of fibers, as best illustrated in FIGURE 3.

While copper fibers are preferred for the mat because of their excellentheat transfer characteristics, it should be appreciated that other typesof metallic fibers can also be employed. It should also be apparent thatthe fibers can be deposited about the tube 11 in any of a variety ofmanners. The simplest consists in simply dropping the fibers by gravityonto and around the tube 11, using the screen 12 as a collector. Inorder that the metallic fibers have a substantial amount of mobilityduring the felting, it is advisable to employ fibers which have lengthsnot in excess of two inches, and preferably not in excess of one inch.Particularly good results have been achieved by employing fibers in therange from one quarter to three quarters inch in length.

Another procedure for depositing the fiber mat 13 about the tube 11consists in suspending the metallic fibers in a liquid medium such asoil or glycerine, agitating the fibers in suspension so that a uniformslurry 18 produced, and then pouring the slurry over the tube 11 so thatthe suspending medium drains out through the screen 12, leaving arandomly oriented felt of fibers about the tube 11. 7

Still another technique which can be employed consists in suspending theshort length fibers in. an air stream under a slight positive pressure,and blowing the fibers Onto and about the tube 11 until a mat ofsufficient thickness is built up.

By any of these means of deposition, the porous, randomly orineted matof fibers can be produced about the tube 11. The best results, theporosity of the mat should be at least 50%, while it may be as high asAfter the fiber mat 16 has been incorporated about the tube 11, a secondscreen 1 4 may be secured across the face of the frame 10 to furtherrigidify the structure 3 without significantly increasing its resistanceto air flow. As illustrated in FIGURE 3, the upper ends a and 10b of theframe 10 may be bent over to provide areas for fastening the screen 14to the frame 10.

After the mat has been built up, the complete assembly is treated toprovide metal-to-metal bonds between the tube 11 and the fibers, as wellas between the fibers themselves. This is most conveniently done bypassing the entire assembly into a sintering furnace and holding theassembly within the furnace, in the presence of a reducing or anon-oxidizing atmosphere until the metal-tometal bonds are produced.Generally, the sintering temperature will be on the order of two-thirdsof the melting temperature of the metal involved, expressed in degreesKelvin.

,After.sintering themetal fibers are. secured bymetallurgical bonds tothe surface of the tube 11 and are similarly secured to adjoining fibersat their areas of contact. Some sintering of fibers also occurs to thematerial of the opposed screens 12 and 14, resulting in the productionof a completely porous but substantially rigid heat transfer assembly.

As previously indicated, another method of securing the metal-to-metalbonds consists in pre-coating the metal fibers with a brazing compound,such as a low melting alloy, and then heating the assembly to atemperature sufficient to melt the brazing compound without melting thefibers or the tube 11. When the molten material has solidified, it formsmetal bonds at the areas along which the fibers contact the surface ofthe tube 11, and also along those areas at which the fibers contact eachother.

A modified form of the invention is illustrated in FIGURE 5 of thedrawings. In this form, the heat exchanger is composed of a pair ofopposed side plates 16 and 17 spaced from each other by means of sheetmetal separators 1 8, 19, 20, 21 and 22, thereby providing a series ofcompartments 23, 24, 2 5 and 26. Metal fibers 27 are disposed in eachcompartment thus provided, the fibers 27 being bonded to each other (bysintering, brazing, or the like) and also being bonded to the walls ofthe compartment which they abut. With the illustrated structure, a hotfluid can be introduced through the fibrous masses in compartments 23and 25 and a cooling fluid through compartments 24 and 26 incountercurrent flow to the hot fluids in the adjoining compartments, andthereby provide eificient heat exchange between the fluid streams.

The fiber mat possesses excellent heat transfer properties from thebonds between the fibers and the tubing, and between the fibersthemselves. The very high porosites achieved by the felting processpermits the easy passage of air or gases through the felted body. Theheat transfer is therefore by conduction through the wall of the tubingfrom the fluid circulated through the tubing, through the high specificsurface fiber network by conduction, and finally to the forcedpermeating gas by convection and radiation.

It should be evident that various modifications can be made to thedescribed embodiments without departing from the scope of the presentinvention.

We claim as our invention:

1. The method of making a heat transfer assembly which comprisespositioning a heat transfer element in spaced relation to a foraminoussurface, dispersing heat conductive metal fibers in three dimensionalrandom orientation to fill up the space between said heat transferelement and said foraminous surface, and thereafter metallurgicallybonding said fibers to said heat transfer element, to said foraminoussurface, and to themselves.

2. The method of making a heat transfer assembly which comprisespositioning a heat transfer element in spaced relation to a foraminoussurface, dispersing heat conductive metal fibers in three dimensionalrandom orientation to fill up the space between said heat transferelement and said foraminous surface, said fibers having .lengths. not.in excess of two inches. and having lengths at least 10 times their meandimension in cross-section, and thereafter metallurgicaly bonding saidfibers to said heat transfer element, to said foraminous surface, and tothemselves.

3. The method of making a heat transfer assembly which comprisespositioning a heat transfer element in spaced relation to a foraminoussurface, dispersing heat conductive metal fibers in three dimensionalrandom orientation to fill up the space between said heat transferelement and said foraminous surface, and thereafter sintering saidfibers to said heat transfer element, to said foraminous surface, and tothemselves.

4. The method of making a heat transfer assembly which comprisespositioning a heat transfer element in spaced relation to a foraminoussurface, dispersing heat conductive metal fibers in three dimensionalrandom orientation to fill up the space between said heat transferelement and said foraminous surface, each of said fibers having a lengthnot in excess of two inches and having a length at least 10 times itsmean dimension in crosssection, and thereafter sintering said fibers tosaid heat transfer element, to said foraminous surface, and tothemselves.

5. The method of making a heat transfer assembly which comprisespositioning a heat transfer element in spaced relation to a foraminoussurface, dispersing heat conductive metal fibers in three dimensionalrandom orientation to fill up the space between said heat transferelement and said foraminous surface, and thereafter metallurgicallybonding said fibers to said heat transfer element, to said foraminoussurface, and to themselves to form a mat of felted fibers having aporosity of at least 50%.

References Cited by the Examiner UNITED STATES PATENTS 1,893,330 1/1933Jones 113118 XR 2,401,797 6/1946 Rasmusson 3,062,509 11/1-9-62 Mulder29157.3 3,127,668 4/1964 Troy 29419 XR JOHN F. CAMPBELL, PrimaryExaminer.

J. D. HOBART, Assistant Examiner.

1. THE METHOD OF MAKING A HEAT TRANSFER ASSEMBLY WHICH COMPRISESPOSITIONING A HEAT TRANSFER ELEMENT IN SPACED RELATION TO A FORAMINOUSSURFACE, DISPSERING HEAT CONDUCTIVE METAL FIBERS IN THREE DIMENSIONALRANDOM ORIENTATION TO FILL UP THE SPACE BETWEEN SAID HEAT TRANSFERELEMENT AND SAID FORAMINOUS SURFACE, AND THEREAFTER METALLURGICALLYBONDING SAID FIBERS TO SAID HEAT TRANSFER ELEMENT, TO SAID FORAMINOUSSURFACE, AND TO THEMSELVES.